Ordinary Portland Cement (OPC, also known as Portland Cement) underpins modern construction technology and constitutes the second most manufactured product in the world. According to Struble et al., Introduction to Portland Cement, in Portland Cement: Composition, Production and Properties, pp. 1-47 (2011 ICE Publishing, 3rd ed.); Herfort et al., The Chemistry of Portland Cement Clinker, 2010 Adv. Cem. Res. 22: 187-194; and Jackson, Portland Cement: Classification and Manufacture, in Lea's Chemistry of Cement and Concrete, pp. 25-94 (2003 Elsevier, 4th ed.), U.S. and world production of OPC currently stands at 72 million metric tons (t) and 2.8 billion t respectively.
OPC manufacturing has the disadvantage of having a large ecological footprint; it requires large amounts of mineral, fuel and water resources, and is accompanied by the release of various pollutants. See, Huntzinger et al., A life-cycle assessment of Portland cement manufacturing, 2009 J. Cleaner Prod. 17: 668-675; Flower et al., Greenhouse gas emissions due to concrete manufacture, 2007 Int. J. LCA 12: 282-288; Marceau et al., Life Cycle Inventory of Portland Cement Manufacture, pp. 1-68 (2006 Portland Cement Association); Rehan et al., Carbon dioxide emissions and climate change: policy implications for the cement industry, 2005 Environ. Sci. Policy 8: 105-114; Worrell, Cement and Energy, in Encyclopedia of Energy, pp. 307-316 (2004 Elsevier, Volume 1); van Oss, and Padovani, Cement manufacture and the environment, Part 2: Environmental challenges and opportunities, 2003 J. Ind. Ecol. 7: 93-126; van Oss and Padovani, Cement manufacture and the environment, Part 1: Chemistry and technology. 2002 J. Ind. Ecol. 6: 89-105; and Worrell, et al., Carbon dioxide emissions from the global cement industry, 2001 Ann. Rev. Energy Env. 26: 303-3029.
Indeed, every 1.0 t of OPC uses 1.50-1.75 t of minerals, 0.89-1.21 t of fuel, and 0.52-1.03 t of water for its production, and results in the emission of 0.70-0.92 t of carbon dioxide as well as various other toxic substances, including carbon monoxide, nitrogen oxides, sulfur oxides, hydrogen chloride, dioxins, lead, mercury and selenium. Since 72 million t of OPC are produced in the U.S. every year, this equates to the use of 108-126 million t of minerals, 64-87 million t of fuel, and 37-74 million t of water, and the emission of 50-66 million t of carbon dioxide.
In view of the poor environmental metrics of OPC, efforts have been made to produce “Green” and sustainable cements by partially or completely replacing OPC with alternative binders with lower ecological footprints, as described in the publications, Schneider et al., Sustainable cement production—present and future. 2011 Cement Concrete Res. 41: 642-650; Sharp, et al., Novel cement systems, 2010 Adv. Cem. Res. 22: 195-202; Damtoft et al., Sustainable development and climate change initiatives, 2008 Cement Concrete Res. 38: 115-127; Phair, Green chemistry for sustainable cement production and use. 2006 Green Chem. 8: 763-780; and Placet et al., Toward a Sustainable Cement Industry, pp 1-92 (2002 Battelle, Substudy 7).
With suitable formulation and additives, OPC can be partially or completely replaced by a variety of minerals and industrial byproducts, collectively referred to as Supplementary Cementitious Materials (SCMs, also known as Mineral Admixtures), the resultant products being referred to as blended cements and non-Portland cements respectively. Blended cements are described in the publications: Massazza, Pozzolana and pozzolanic cements, in Lea's Chemistry, of Cement and Concrete, pp. 471-631 (2003 Elsevier, 4th ed.); and Mineral Admixtures in Cement and Concrete, pp. 1-248 (2011 CRC Press); and Kosmatks et al., Portland, Blended, and Other Hydraulic Cements, in Design and Control of Concrete Mixtures, pp. 21-56 (2006 Portland Cement Association, 14th ed.).
SCMs are characterized as cementitious, pozzolanic (having cementitious activity in the presence of lime), or both cementitious and pozzolanic. ASTM C-989, C-618 and C-1240 describe three major classes of SCMS: (i) Iron blast furnace slags (cementitious); (ii) Fly ash Class C (cementitious and pozzolanic), fly ash Class F (pozzolanic), and natural (raw or calcined) pozzolans such as diatomaceous earths, opaline cherts and shales, tuffs, volcanic ashes, and calcined clays and shales; and (iii) Silica fume (pozzolanic). The major SCMs are low-value industrial byproducts, most notably fly ash and blast furnace slag, which are waste products of coal-fired power plants and iron ore smelters respectively. SCMs are described in Mineral Admixtures in Cement and Concrete, pp. 1-248 (2011 CRC Press); Supplementary Cementing Materials, pp. 1-283 (2011 Springer); Lohtia, et al., Mineral Admixtures, in Concrete Admixtures Handbook, pp. 657-739 (2006 Noyes, 2nd ed.); Waste Materials and Byproducts in Concrete, pp. 1-407 (2008 Springer), Kosmatks et al., Fly Ash, Slag, Silica Fume, and Natural Pozzolans, in Design and Control of Concrete Mixtures, pp. 57-72 (2006 Portland Cement Association, 14th ed.); Massazza, Pozzolana and pozzolanic cements, in Lea's Chemistry of Cement and Concrete, pp. 471-631 (2003 Elsevier, 4th ed.); and Waste Materials Used in Concrete Manufacturing, pp. 1-637 (1997 Noyes).
Since SCMs are industrial byproducts or lightly processed minerals, they have lower ecological footprints than OPC, as described by Schneider et al., Sustainable cement production—present and future, 2011 Cement Concrete Res. 41: 642-650; Sharp et al., Novel cement systems, 2010 Adv. Cem. res. 22: 195-202; Damtoft et al., Sustainable development and climate change initiatives, 2008 Cement Concrete Res. 38: 115-127; Phair, Green chemistry for sustainable cement production and use, 2006 Green Chem. 8: 763-780; and Placet et al., Toward a Sustainable Cement Industry, pp 1-92 (2002 Battelle, Substudy 7). For example, the CO2 footprints of fly ash, blast furnace slag, steel slag, limestone fines and calcined natural pozzolans are 0.02-0.04, 0.13-0.16, 0.10-0.14, 0.16-0.19 and 0.18-0.22 t-CO2 per t-SCM respectively, compared to OPC's CO2 footprint of 0.70-0.92 t-CO2 per t-cement. Substituting SCMs for OPC proportionally improves environmental metrics, with blended cements and non-Portland cements showing CO2 footprints of only 0.28-0.67 and 0.26-0.50 t-CO2 per t-cement respectively.
Blended cements, especially those employing fly ash and/or blast furnace slag as SCMs, are used worldwide in place of OPC for certain applications. In the U.S., blended cements are specified under ASTM C-595-06 as Portland-Pozzolan cement, Type IP(X), which allows 0-40% w/w replacement of OPC with a pozzolan (typically fly ash), and as Portland-blast furnace slag cement, Type IS(X), which allows 0-95% w/w replacement of OPC with blast furnace slag.
Blast furnace slag is a cementitious material that can replace up to 95% w/w of OPC in blended cements, and is typically used at 50% replacement level. But, its availability is limited, with U.S. and world production standing at 9 million and 250 million t respectively, or only 13 and 9% of OPC consumption respectively. Fly ash is available in much larger quantities, with U.S. and world production standing at 70 million and 700 million t, or on par with U.S. OPC consumption and some 25% of world OPC usage respectively. Its use in blended cements at low to moderate levels (0-50% replacement of OPC) can improve durability metrics such as ultimate compressive strength, permeability, salt resistance, rebar corrosion resistance, and freeze-thaw resistance. Fly ash is available at a low cost—in the range of $5-60 per t in the U.S., and in quantities large enough to satisfy the total demand for cement in the U.S. and some 25% of world cement demand.
Despite its availability and use potential, fly ash is highly underutilized with less than 15% of U.S. ash output going into blended cements, and with some 65% of ash sent for storage or disposal. This has resulted in the accumulation of over 500 million t of ash, a situation that seems certain to worsen as U.S. ash output is projected to increase to 100 million t by 2030. Unused ash carries significant public health and environmental burdens due to the presence of potentially toxic elements, and approaches for its safe and beneficial utilization in added value products such as cement are essential for the sustainable management of ash. Fly ash is reviewed in the publications, Ahmaruzzaman, A review on the utilization of fly ash, 2010 Prog. Energy Combust. Sci. 36: 327-363); Kelly et al., Coal Combustion Products Statistics, pp. 1-2 (2010 U.S. Geological Survey); Coal Combustion Product Production & Use Survey Results, pp. 1-2 (2009 American Coal Ash Association); Beneficial Use of Secondary Materials—Coal Combustion Products, pp. 1-95 (2008 U.S. Environmental Protection Agency); Nugteren, Fly ash: From waste to industrial product, 2007 Part. Part. Sys. Charact. 24: 49-55; Hall et al., Fly ash quality, past, present and future, and the effect of ash on the development of novel products, 2002 J. Chem. Technol. Biotechnol. 77: 234-239; and Rayzman, et al., Technology for chemical-metallurgical coal ash utilization, 1997 Energy Fuels 11: 761-773.
While fly ash is an attractive SCM, performance considerations limit the fly ash content in blended cements to less than 50% replacement of OPC, with U.S. states allowing a maximum substitution level in the range of just 15-25%. Despite the use of performance-enhancing chemical additives, the replacement of more than 25% of OPC with fly ash retards setting and the rate of hardness and compressive strength development, increases the demand for air-entraining agents, can adversely affect durability metrics such as salt resistance, and may require the use of increased cement content in cement compositions. For example, blended cements with 25% and 50% w/w replacement of OPC with fly ash typically show only 50-75% and 30-50% of the hardness and compressive strength development of OPC-based compositions, and can require 1-3 months to reach performance parity, as described in the publications, Mineral Admixtures in Cement and Concrete, pp. 1-248 (2011 CRC Press); Supplementary Cementing Materials, pp. 1-283 (2011 Springer); Lohtia et al., Mineral Admixtures, in Concrete Admixtures Handbook, pp. 657-739 (2006 Noyes, 2nd ed.); Waste Materials and Byproducts in Concrete, pp. 1-407 (2008 Springer), Kosmatks, et al., Fly Ash, Slag, Silica Fume, and Natural Pozzolans, in Design and Control of Concrete Mixtures, pp. 57-72 (2006 Portland Cement Association, 14th ed.); Massazza, Pozzolana and pozzolanic cements, in Lea's Chemistry of Cement and Concrete, pp. 471-631 (2003 Elsevier, 4th ed.); and Waste Materials Used in Concrete Manufacturing, pp. 1-637 (1997 Noyes).
Attempts to develop blended cements containing other SCMs such as limestone, steel slag and non-ferrous slags, have been hampered by similar albeit worse problems to OPC-fly ash cements. Thus, ASTM C-150 allows the addition of only 5% of limestone to OPC, due to concerns about setting retardation, poor strength development and excessive shrinkage at higher limestone contents. Blended cements made with steel slag and non-ferrous slags face a variety of problems including poor setting, slow hardness and strength development, delayed expansion, cracking, and poor freeze-thaw resistance.
Performance issues and the lack of industry-standard specifications for high-SCM content blended cements necessitate extensive pretesting to ensure durable cement compositions that meet building codes, and invariably lead to higher costs. As such, the availability and use of blended cements in the U.S. has been highly restricted, with the average replacement level of SCMs for OPC being less than 15%, and less than 3% of cement in the U.S. being blended cement. See e.g. Loreti, Greenhouse Gas Emission Reductions from Blended Cement Production, pp. 1-35 (2008 The Loreti Group); and Characteristics of Portland and Blended Cements: Results of a Survey of Manufacturers, pp. 83-101 (2006 IEEE Conference Proceedings).
Non-Portland cements, such as those based upon fly ash and/or blast furnace slag suffer from similar albeit worse problems to blended cements. For example, cements made from fly ash, or blends of fly ash with other SCMs, can display flash or delayed setting, poor hardness and compressive strength development, excessive shrinkage, aggregate-induced expansion, and poor long-term durability. Indeed, there is no industry-standard specification for non-Portland cements in the U.S., and no significant commercial production of such cements.
Various approaches have been used to elevate the performance of blended cements and non-Portland cements to that of OPC, the most successful being the use of chemical admixtures to improve such properties as setting, and the rate of hardness and compressive strength development. As generally described in the publications, Chemical Admixtures for Concrete, pp. 1-1 to 6-10 (1999 Taylor and Francis, 3rd ed.) and Concrete Admixtures Handbook, pp. 137-1024 (1996 Noyes, 2nd ed.), chemical admixtures are inorganic or organic compounds that are used to modify the physicochemical properties of cement compositions. These properties include viscosity, slump, water requirement, setting, hardness, compressive strength, shrinkage, porosity, air content, water resistance, freeze-thaw resistance, salt resistance and rebar corrosion resistance. Chemical admixtures such as sodium hydroxide, sodium silicate, sodium aluminate, sodium citrate, calcium chloride, calcium sulfate, calcium nitrate, calcium nitrite, calcium formate, citric acid, carboxylic acid-containing polymers and copolymers, and alkanolamines have long been used to regulate setting and hardness and compressive strength development in OPC, and have found similar albeit less utility in blended cements and non-Portland cements.
The prior art describes the application of calcium salts, citric acid, and citrate salts as accelerators for high-SCM content blended cements and SCM-based non-OPC cements. However, these approaches are often not placement/use equivalent to OPC, and give variable results in terms of setting, hardness and strength development, shrinkage, cracking, aggregate reactivity and/or long-term durability. Because of these issues, none of these approaches have found significant commercial applications, other than very specialized uses such as rapid-setting cements. The use of citric acid and citrate salts as accelerator(s) for blended cements and fly ash-based non-Portland cements is described U.S. Pat. Nos. 4,842,649, 4,997,484, 5,374,308, 5,536,310, 4,640,715, 4,642,137, 7,854,803, 7,288,148, U.S. Patent Application 2008178770A1 and U.S. Pat. No. 6,827,776. The use of calcium sulfate as accelerator for fly ash-based non-Portland cements is described in the publications, U.S. Pat. Nos. 5,439,518, 5,578,122, 4,240,952, 4,470,850 and 4,256,504. The use of calcium nitrate as an accelerator for blended cements is described in the publication, U.S. Patent Application 20110048285 A1.
The prior art also describes the use of highly alkaline chemical admixtures (also known as alkali activators) such as sodium hydroxide, potassium hydroxide, sodium silicate and sodium aluminate for forming so-called “geopolymers” with blended cements and SCM-based non-Portland cements. While the properties of geopolymer cements can match or exceed those of OPC or conventional blended cements, they have severe drawbacks—most notably that the admixtures are highly corrosive and required in large amounts (typically 5-25% of the cement content), that the cements are not use/placement equivalent to OPC and require special formulation, handling and curing, and that they lack long-term stability and durability. Indeed, there is currently no geopolymeric cement of commercial significance in the U.S. Alkali activators and geopolymer cements are described in the publications, Alkali Activated Cements and Concretes, pp. 1-334 (2006 Taylor and Francis); Duxson et al., The role of inorganic polymer technology in the development of ‘green concrete’, 2007 Cement Concrete Res. 37: 1590-1597; Pachecho-Torgal et al., Alkali-activated binders: A review Part 1. Historical background, terminology, reaction mechanisms and hydration products, 2008 Constr. Build. Mater. 22: 1305-1314; Pachecho-Torgal et al., Alkali-activated binders: A review. Part 2. About materials and binders manufacture, 2008 Constr. Build. Mater. 22: 1315-1322; and Duxson et al., Geopolymer technology: the current state of the art, 2007 J. Mater. Sci. 42: 2917-2933.
While currently available chemical admixtures are useful for improving the properties of blended cements containing low to moderate levels of SCMs (up to 25% of fly ash, up to 50% of blast furnace slag, and up to 15% of limestone), their performance is not as satisfactory with higher levels of SCMs, and even less so with SCM-based non-Portland cements. To date there is no commercial general-use high-SCM content blended cement or non-Portland cement available in the U.S., despite the demand for such Green and sustainable cements as alternatives to OPC.